Magnetic storage systems, such as hard disk drives (HDDs), are used as mass storage in a wide variety of devices, including but not limited to personal computers, digital versatile disc (DVD) players, high definition television (HDTV) receivers, vehicle control systems, cellular or mobile telephones, television set top boxes, and portable media players. As these magnetic storage systems become smaller and/or attain higher data storage capacities, the density of data on the magnetic storage medium becomes higher.
A typical HDD includes magnetic storage media of one or more flat disks, called platters (sometimes also “disks” or “discs”). The platters are generally formed of two main substances: a substrate material that gives it structure and rigidity, and a magnetic media coating which holds the magnetic impulses (or moments) that represent data. A typical HDD further includes a read/write head, generally a magnetic transducer which can sense and/or change the magnetic fields stored on the platters. The read/write head is attached to a slider, generally an armature capable of placing the read/write head at a desired location over the platter.
The areal recording density of magnetic storage media, particularly of hard disk drives (HDDs), has increased greatly over time, and currently reaches several Gb/in2 or more. Such improvement in areal density derives largely from innovation and improvement of various elemental technologies such as use of magneto-resistive storage systems, use of recording magnetic pole materials having high saturation magnetic flux densities, improvements in the manufacturing of magnetic sensor heads with a narrow track width, use of magnetic sensor heads having a narrower gap between the head the magnetic storage medium, miniaturization and high-precision processing of slider mechanisms, high-precision servo technology, and development of novel modulation/demodulation technologies. In addition, with respect to magnetic storage media, there have been advances in elemental technologies such as smoothing and flattening of the media surface (facilitating low flying height operation of the magnetic sensor head over the medium), reduction in magnetization transition width due to increases in coercivity and decreases in the thickness of magnetic layers, and medium noise reduction due to decreases in exchange interaction between magnetic grains and reductions in magnetic grain size.
As the density of data on the magnetic storage medium increases, the strength of the magnetic fields generally decrease, in order to minimize interference. Higher areal density in magnetic storage media generally requires advanced read/write transducer head design with compatible advanced preamplification circuit architectures. Advanced read head technologies such as giant magneto-resistive (GMR), tunneling magneto-resistive (TuMR), and current perpendicular to plane (CPP) with decreasingly small nano-scale dimensions require very precise electronic biasing to function properly and reliably. Due to the very weak signals detected by these read head sensors, extreme attention has to be paid to noise and other errors introduced by the preamplification system.
Referring now to FIG. 1A, a conventional differential magnetic storage reading circuit 100 having current bias generators 102a and 102b is shown. MR sensor 101 is a magneto-resistive read/write transducer (e.g., positioned over a magnetic storage medium by a slider/armature apparatus, not shown). Variable current bias generator 102a provides a bias current at both nodes 103a and 103b of MR sensor 101. MR sensor 101 may be coupled to amplifier 110, where changes in the resistance of the MR sensor (RMR) caused by changes in the magnetic field on the storage medium are amplified to read data from the storage medium. Similarly, FIG. 1B shows a conventional differential magnetic storage reading circuit 100 having voltage bias generators 105a and 105b. 
Two types of basic biasing schemes are currently used in conventional preamplifiers. Referring now to FIG. 2A, a conventional current bias block 200 is shown. Current bias block 200 may comprise current source 202, configured to provide a desired current to a node of MR sensor 203. In differential circuit applications, two bias blocks may be used to provide a bias current at each end of MR sensor 203. The current source 202 may comprise, for example, a current digital-to-analog convert (IDAC), such as a conventional 10-bit IDAC. The advantages of a current bias circuit include simple implementation and programming resolution that is independent of RMR.
Referring now to FIG. 2B, a conventional voltage bias block 210 is shown. Voltage source 213 provides bias voltage VBIAS to MR sensor 214. Comparison circuit 212 compares VBIAS to a desired (programmed) voltage 211, and adjusts voltage source 213 accordingly to maintain the desired voltage. A conventional voltage bias block advantageously provides bias voltage and programmed resolution of the bias voltage that are independent of the resistance of the MR sensor (RMR). However, the voltage bias block generally produces larger bias noise than other biasing schemes, and requires a relatively complicated implementation.
As the areal density of magnetic storage devices increases, and the strength of the magnetic field on such devices decreases, the voltage and/or current bias applied to the MR sensor becomes smaller and must be more finely controlled. Thus, the influence of feedback resistors (e.g., feedback resistors 111a and 111b) on the MR sensor bias becomes more significant. Therefore, it would be advantageous to calibrate the MR sensor bias to account for any error introduced by feedback resistors.